It is known to make copies of an image by a photographic means using a light-sensitive composition composed of a mixture of a polymerizable compound having an ethylenically unsaturated bond, a photopolymerization initiator, and further, if necessary, a suitable binder having a film-forming faculty and a thermal polymerization inhibitor. That is, as described in U.S. Pat. Nos. 2,927,022, 2,903,356, and 3,870,524, since this kind of light-sensitive composition can be hardened and insolubilized by the irradiation of light, a desired hardened image of a photopolymerizable composition can be formed by forming a proper layer or film using the light-sensitive composition, irradiating the layer or film by light through a negative of a desired image, and then removing only the unexposed portions with a proper solvent. The light-sensitive composition of this type is as a matter of course very useful for making light-sensitive printing plates, photoresists, etc.
Hitherto, since a polymerizable compound having an ethylenically unsaturated bond does not show a sufficient light sensitivity by itself only, it has been proposed to add thereto a photopolymerization initiator for increasing the light sensitivity thereof and as such a photopolymerization initiator, there are benzyl, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, 2-ethylanthraquinone, etc.
However, the use of such a photopolymerization initiator for a photopolymerizable composition encounters such problems that since the photopolymerizable composition has a low respondence for hardening and a long light exposure time is required for forming images, an image having good image quality is not reproduced even by the existence of slight vibration in operation in the case of reproducing precise images and also since the amount of energy of a light source for light exposure must be increased, it is required to take a counterplan to the dissipation of a large amount of heat generated by the increase of the light exposure energy and further the layer or film of the composition is liable to deform or change quality. Therefore, it has been desired to develop a photopolymerization initiator without having the above-described problems.
Also, these conventional photopolymerization initiators show very low photopolymerizing faculty for a light source of a visible light region of 400 nm or more as compared to the photopolymerizing faculty for a light source of a ultraviolet region of 400 nm or less in wavelength and accordingly the applicable range of the photopolymerizable compositions containing them is greatly limited or narrowed.
Hitherto, various attempts have been proposed in regard to a photopolymerization system responding visible light. For example, U.S. Pat. No. 2,850,445 describes that certain kinds of photoreducible dyes such as, for example, Rose Bengale, Eosine, erythrosine, etc., have an effective visible light respondence. Thereafter, as the improvements of the above-described techniques, there are proposed a polymerization initiation system composed of a dye and an amine (e.g., Japanese Patent Publication No. 20189/69, corresponding to British Pat. Nos. 1,135,280, 1,146,497 and 1,146,498), a system composed of a hexaarylbiimidazole, a radical generating agent, and a dye (e.g., Japanese Patent Publication No. 37377/70, corresponding to U.S. Pat. No. 3,479,185), a system composed of a hexaarylbiimidazole and a p-dialkylaminobenzylidene ketone (e.g., Japanese Patent Application (OPI) Nos. 2528/72, corresponding to U.S. Pat. No. 3,652,275, and 155292/79, corresponding to U.S. Pat. No. 4,162,162), and a system composed of a substituted triazine and a merocyanine dye (e.g., Japanese Patent Application (OPI) No. 151024/79). The term "OPI" as used herein refers to a "published unexamined Japanese patent application".
These effective may be effective for visible light, but the light-sensitive speed of these systems is not yet satisfactory and hence a further improvement has been desired.